Modern oil and gas explorations are carried out in complex geologic settings. The drilling operations rely heavily on real time information on the status of the drilling tools as well as the earth formation properties. The information is obtained using sensors and measurement-while-drilling (MWD) and logging-while-drilling (LWD) instruments. Although MWD refers to the measurement of the movement and location of the drilling assembly while the drilling continues and LWD focuses more on the measurement of formation properties, they are used interchangeably in this disclosure. MWD/LWD instruments are often installed in the drill collar inside the drilling assembly (i.e., bottom hole assembly or BHA).
Alternative to or complimentary to the MWD/LWD instruments, wireline logging may be used to examine the earth formation. Typically, after the drill string is removed from the borehole, a sonde is lowered to the bottom of the region of interest and subsequently pulled upward. On the upward trip, the sonde measures the properties of the formation along its path.
Sensors are employed to obtain measurements in both the MWD/LWD instruments and the wireline logging approach. Other electronic components include active components, such as printed circuit board assemblies (PCBA) and transistors, or passive components, such as resistors and capacitors.
PCBAs are used throughout the drilling system. For example, they can be used in the operation of the power supply, temperature sensors, pressure transducers, the battery, etc. PCBAs such as the master memory board, the read out board, the transmitter or a receiver board, and the accelerometer board are among electronic devices commonly used in a downhole environment.
PCBAs can be coupled to various sensors in a drilling system by known methods. In some embodiments, sensors may be integrated on a master memory board. Sensors can be measurement sensors that monitor real-time conditions during a drilling process. For example, some sensors monitor the properties of earth formation such as resistivity, density, porosity, permeability, acoustic properties, nuclear-magnetic resonance properties, corrosive properties of the fluids or formation, and salt or saline contents. Other sensors monitor the condition of drilling tools, such as vibrations (lateral, torsional, axial, etc.), orientations, and accelerations in the drill bit or drill strings. Still other sensors monitor the temperature and pressure of the downhole environment. In other embodiments, sensors may be prognostic sensors. Prognostic sensors are subject to more severe conditions than in a typical drilling operation (e.g., higher temperature or pressure) so that they fail at an accelerated rate. They could be used to estimate the time of failure of another component.
Sensors can be mounted on any other suitable components in a drilling assembly. For example, they can be attached to a drill bit to monitor its movement or temperature. Sensors can also be mounted along the borehole, for example, to monitor the pressure or flow rate of the drilling mud along its path. Sensors (e.g., RFID) can even be put into the fluid in the drilling system and be dispersed into the earth formation.
A processor usually is installed on a PCBA. The processor is configured to receive, store, or execute data such as computer codes or sensor signals. For example, a processor can be coupled to a program module which supplies executable instructions and a recording medium that stores various results of calculations performed by the processor. Sensor signals are the input to the processor. PCBAs may also contain telemetry unit so it can transmit sensor signals to a surface instrument for further processing. Conversely, a PCBA may also receive inputs from a surface control unit.
Downhole drilling tools, especially the drilling assembly (a.k.a. BHA), are subject to high temperatures and pressures. Conventional drilling may occur at a temperature up to 125° C. In contrast, bottom hole temperature may exceed 200° C. in deep wells. Such high temperatures drastically reduce the reliability and useful life of downhole electronic components due to leakage current, material degradation, outgassing, corrosion, etc. As a rule of thumb, for every 25° C. increase in temperature, an electronic component would lose 90% of its useful life. Therefore, there are great needs for electronic components that have an adequate operable life, e.g., from a few days to a few weeks, for high temperature drilling operations.